The production of ohmic contacts to semiconductors is of constant concern in semiconductor device manufacture. The contacting of compound semiconductors has proven particularly difficult, perhaps, because of the more complex chemistry of these materials relative to elemental semiconductors. A survey of representative technology in this area appears in Solid State Electronics, 18 (1975) 541. The contacting procedures typically involve the provision of two types of constituents. The first is a dopant for the semiconductor and the second is a metal for providing external contact. N-type dopants for Group III-V semiconductors include tin and germanium. A widely used contacting metal is gold. The use of such gold-tin and gold-germanium contacts requires considerable care to avoid the balling up of these constituents on the semiconductor surface and other types of nonuniformity (U.S. Pat. No. 3,890,699, issued June 24, 1975, see, for example, Column 1, lines 13-17). This reference teaches the sequential deposition of layers of germanium, silver, and gold. In another prior art method (U.S. Pat. No. 3,959,522 issued May 25, 1976) gold is first deposited on a heated substrate, the substrate is cooled, a layer of tin is deposited on the cooled substrate, the device is reheated, then cooled again to receive deposits of nickel and gold for external contact. In a multilayer process developed for contacting p-type materials zinc is used as a p-type dopant. However, zinc is known to have a high vapor pressure. In an effort to suppress the evaporation of zinc during the heat treatment step needed to diffuse the zinc into the semiconductor, a method involving the sequential deposition of a layer of gold followed by a layer of a gold-zinc alloy and a second layer of gold was developed. (U.S. Pat. No. 3,850,688 issued Nov. 26, 1974).
Of the Group III-V semiconductors those containing aluminum (e.g., gallium aluminum arsenide) have posed a particular contacting problem. It is believed that the presence of aluminum compounds in the surface tends to make these contacts electrically unreliable. A method developed to counteract this problem is disclosed in U.S. Pat. No. 4,081,824, issued Mar. 28, 1978. This method involves the deposition of a transition layer of aluminum and the dopant element followed by a layer of gold. This method has proven useful in manufacture in providing a high yield of electrically reliable contacts. However in exemplary manufacturing runs a significant number of contacts experienced mechanical failure after wires were bonded to the deposited contact pads. When the wires were stressed a significant number of contacts separated at the interface between the semiconductor and deposited contact material. Investigation of these devices by electron microscopy showed the presence of voids in the interface. The presence of these voids is believed to be responsible for the bonding failures. It is postulated that these voids are produced by the diffusion of tin during and after gold deposition but prior to heat treatment. The provision of a contact which is electrically and mechanically reliable has been of constant concern.